基于粒子群算法的Pt-Pd合金纳米粒子的稳定结构研究

Pt-Pd合金纳米粒子相对于Pt及Pd单晶纳米粒子均具有更好的催化活性和选择性, 研究它的稳定结构对进一步了解催化性能具有重要意义. 本文采用粒子群算法和量子修正Sutton-Chen多体势对不同尺寸、 不同组成比例的二十四面体Pt-Pd合金纳米粒子的结构稳定性进行研究. 结果表明: Pt-Pd合金纳米粒子中Pt原子趋向于分布在纳米粒子内层, 而Pd原子趋向于分布在纳米粒子外层, 且Pt, Pd原子的分布越对称有序, 其能量越低, 结构越稳定; 随着Pt原子比例的增加, 三种不同尺寸的合金纳米粒子的Warren-Cowley化学短程有序值都逐渐升高, 即纳米粒子更趋向于偏聚分布状态; 在相同比例下, 小尺寸纳米粒子的偏聚程度比大尺 寸纳米粒子的大. 关键词： 合金纳米粒子 粒子群算法 稳态结构     

高指数晶面Au-Pd纳米合金粒子的稳定结构研究

基于蒙特卡罗方法, 本文采用了紧束缚势和量子修正Sutton-Chen型多体势两种势能函数对具有不同比例、不同尺寸二十四面体Au-Pd合金纳米粒子的稳定结构、表面原子分布、核壳分布和化学短程序值进行了研究分析. 结果表明: 两种势函数得到的表面原子分布趋势一致, 即Au-Pd合金纳米粒子中的Au原子趋向于分布在纳米粒子的外层, 而Pd原子趋向于分布在纳米粒子的内层, 这有利于降低纳米粒子的总能; 在Au原子比例较小时, 两种势函数下得到的稳定结构均呈现出核壳分离的结构, 随着Au比例的增大, 紧束缚势函数下得到的纳米粒子稳定结构将趋向于洋葱状的多壳层的结构; 相比于紧束缚势, 量子修正Sutton-Chen型多体势作用下得到的Au-Pd纳米粒子的稳定结构偏聚程度更高.     

基于内核构建的Cu-Au-Pd团簇稳定结构优化

具有特殊催化、磁性和化学活性的三元合金团簇已成为基础科学研究的热点问题.确定其稳定结构是研究团簇性质的重要前提.针对大尺寸Cu-Au-Pd团簇结构优化,提出了内核构建的方法改进了自适应免疫优化算法的效率(称为AIOA-IC算法).采用基于紧束缚势二阶矩近似的多体Gupta势函数来描述三元合金团簇原子间相互作用.为测试算法效率优化了原子数为60的Ag-Pd-Pt团簇稳定结构.结果显示新得到的结构比文献报道的团簇结构势能量值更低,由此可知AIOA-IC算法具有更强的势能面搜索能力.运用该算法研究了38及55原子Cu-Au-Pd团簇的稳定结构.所研究的38原子Cu-Au-Pd团簇包含了五折叠、六折叠和截角八面体结构,并且原子成分比例影响了团簇的结构类型.而55原子Cu-Au-Pd团簇均为完整二十面体结构,序列参数显示Cu,Au和Pd原子分层现象明显.对于147原子Cu_(12)Au_(93)Pd_(42)团簇完整二十面体结构,中心原子为Au,内层和次外层分别被12个Cu原子和42个Pd原子占据,最外层则被92个Au原子占满.通过原子半径及表面能分析了Cu,Pd和Au原子分别倾向于分布在内层、次外层和最外层的规律.     

Ag-Au二元纳米微粒吸收谱的计算

纳米微粒的光学性能与其表面等离子体共振关系密切.本文利用推广的Mie理论计算研究了Au-Ag体系单质、合金以及核壳结构纳米颗粒的消光、吸收和散射的性能(包括壳核结构Ag-Au微粒在紫外-可见光的吸收性能),计算结果与实验值相符合得很好.研究表明,随着粒径的增加,微粒表面等离子体共振偶极吸收峰出现红移,波峰位置与纳米微粒的尺寸具有线性关系.壳核结构中,粒径与核壳比决定了整个微粒的吸收性能.进一步研究表明,当Au壳层较薄时,可以获得具有可调光学性能的壳核纳米结构;而当Au壳层较厚时,其光学性能与同尺寸单质Au微粒一致.通过计算分析,本文还将Mie理论推广到具有空腔结构并且壳层厚度达到一定值的纳米微粒.另外,研究发现合金结构纳米微粒的吸收峰位置与合金成分有着线性关系.本研究表明,人们可以通过控制纳米微粒的尺寸、形貌和结构,调节其表面等离子体共振峰位,这大大拓展了纳米微粒的应用范围.     

递归法研究Pt对Ti合金腐蚀影响

采用递归法计算了Ti及Ti合金的电子态密度、环境敏感镶嵌能、费米能级和格位能等电子结构参量.计算发现Pt在晶体中环境敏感镶嵌能和格位能高于表面,从电子层面证实Pt易在 Ti合金表面偏聚.偏聚在表面的Pt有序能为正值,故Pt以有序相（Pt与Ti的化合物）形式分布在合金表面.晶体表面Pt 与Ti的化合物电极电位较低,它与Ti形成微电池.在腐蚀介质的作用下,Pt与Ti的化合物分解,Pt沉淀到晶体表面造成Pt在合金表面形成凹凸不平的Pt电催化层.Pt电催化层加强Ti钝化作用,从而提高了Ti合金的抗腐蚀能力. 关键词： 电子结构 Ti合金表面 钝化     

金属纳米颗粒在Lu_2O_3薄膜中的应变场分析

利用有限元分析法对镶嵌在Lu2O3薄膜中的Au、Cu、Pt、Co金属纳米颗粒的应变场分布进行分析.分析表明:金属纳米颗粒在生长过程中受Lu2O3薄膜的压缩应力作用,从而在纳米颗粒内部和表面产生相应应变,应变分布与金属纳米颗粒的杨氏模量和泊松比有关.杨氏模量大的金属纳米颗粒表面应变和内部应变差异较大;而杨氏模量小的金属纳米颗粒内外应变差相对较小.随着金属纳米颗粒在基体材料内部不断生长,其受到的偏应变也逐渐增大.金属纳米颗粒生长过程中的这种偏应变的存在和变化将极大地影响其内部晶格结构和表面形貌,进而影响金属纳米颗粒的性能.     

包埋Pt纳米粒子对金属-半导体-金属结构ZnO紫外光电探测器性能的影响

利用射频磁控溅射设备制备ZnO薄膜, 最终制备ZnO/Pt纳米粒子/ZnO 结构的金属-半导体-金属型紫外光电探测器. 研究了Pt纳米粒子处在ZnO薄膜层中的不同深度对金属-半导体-金属型紫外光电探测器响应性能的影响. 结果表明, 探测器的响应度随着Pt纳米粒子在ZnO薄膜层中所处深度的增大而升高. 在60 V偏压下, 包埋Pt最深的探测器在波长365 nm处取得响应度最大值1.4 A·W^-1, 包埋有Pt探测器的响应度最大值为无Pt 纳米粒子探测器响应度最大值的7倍. 结合对ZnO薄膜表面的表征及探测器各项性能的测试, 得出包埋Pt纳米粒子增强器件的响应性能可归因于表面等离子体增强散射.     

碳纳米管表面金纳米颗粒的形成与结构转变

利用分子动力学模拟研究了室温下金纳米颗粒在碳纳米管表面的结构和作用能.研究结果表明,金纳米颗粒随着尺寸的增大会发生不同于孤立状态下的结构转变.当原子数小于130时,颗粒属于无序结构;当原子数大于140时,呈现面心立方晶体结构.小金纳米颗粒和碳纳米管结合紧密,相互作用能正比于面对碳纳米管的颗粒表面面积. 关键词： 金纳米颗粒 碳纳米管 分子动力学模拟     

Cu偏析诱导Co团簇结构及性质异常的动力学模拟

采用分子动力学结合镶嵌原子势方法,模拟研究了Cu原子分别分布于基体Co团簇内层和表面构成Cu-Co合金团簇的结构和热力学性质,研究表明,相同数目的 Cu原子掺杂到基体中因掺杂层的不同,会诱导内层Co团簇和外层Co团簇结构、能量及熔点表现出巨大差异;Cu原子在团簇各层掺杂位置的差异,会导致原子向低能态位置偏移,但相对移动后后续原子的补位,使团簇结构随温度呈相对无扩散度相变;Cu原子由内层向表面偏析是内层Co团簇与相同原子数比例的外层Co团簇熔点产生巨大差异的主要原因.     

DNA诱导金纳米粒子自组装及其SERS表征

首先将巯基DNA分子与金纳米粒子偶联,并用琼脂糖凝胶电泳分离出含不同DNA分子数目的金纳米粒子,最后将修饰有互补DNA链的Au纳米粒子进行组装,得到组装体(五聚体)。透射电子显微镜(TEM)研究表明,DNA-Au纳米组装体被成功地获得;表面增强拉曼光谱(SERS)研究表明,与未组装的金纳米粒子相比,DNA-Au纳米组装体具有更强的SERS活性。     

Structural optimization and segregation behavior of quaternary alloy nanoparticles based on simulated annealing algorithm

Alloy nanoparticles exhibit higher catalytic activity than monometallic nanoparticles, and their stable structures are of importance to their applications. We employ the simulated annealing algorithm to systematically explore the stable structure and segregation behavior of tetrahexahedral Pt–Pd–Cu–Au quaternary alloy nanoparticles. Three alloy nanoparticles consisting of 443 atoms, 1417 atoms, and 3285 atoms are considered and compared. The preferred positions of atoms in the nanoparticles are analyzed. The simulation results reveal that Cu and Au atoms tend to occupy the surface, Pt atoms preferentially occupy the middle layers, and Pd atoms tend to segregate to the inner layers. Furthermore, Au atoms present stronger surface segregation than Cu ones. This study provides a fundamental understanding on the structural features and segregation phenomena of multi-metallic nanoparticles.     

GPU-based DPSO algorithm for structural optimization of Pt-Co bimetallic nanoparticles

Alloy nanoparticles (NPs) are potential candidates for catalysts in fuel cells applications, and their physical properties are associated with the corresponding stable structures. In this work, a GPU-based discrete particle swarm optimization (DPSO) method is designed to investigate the stable structure of Pt-Co alloy NPs. Register and an optimal block design are used to further increase the acceleration ratio. Comparative experiments on CPU and GPU show that the GPU-based DPSO algorithm possesses superior computational performance, and Register has a great influence on the acceleration ratio. Analyses on the stable structures of Pt-Co NPs demonstrate that Co atoms preferentially locate at vertex and edges of surface, yet Pt atoms exhibit a strong surface and sub-surface segregation only for their composition over 65%.     

Particle swarm optimization of the stable structure of tetrahexahedral Pt-based bimetallic nanoparticles

Bimetallic nanoparticles, enclosed by high-index facets, have great catalytic activity and selectivity owing to the synergy effects of high-index facets and the electronic structures of alloy. In this paper, a discrete particle swarm optimization algorithm was employed to systematically investigate the structural stability and features of tetrahexahedral Pt-based bimetallic nanoparticles with high-index facets. Different Pt/Ag, Pt/Cu, Pt/Pd atom ratios and particle sizes were considered in this work. The simulation results reveal that these alloy nanoparticles exhibit considerably different structural characteristics. Pt–Ag nanoparticles tend to form Pt–Ag core–shell structure. Pt–Cu nanoparticles are preferred to take multi-shell structure with Cu on the outer surface while Pt–Pd nanoparticles present a mixing structure in the interior and Pd-dominated surface. Atomic distribution and bonding characteristics were applied to further characterize the structural features of Pt-based nanoparticles. This study provides an important insight into the structural stability and features of Pt-based nanoparticles with different alloys.     

On-surface and sub-surface oxygen on ideal and reconstructed Cu(1 0 0)

In order to understand the first steps of the Cu(1 0 0) oxidation we performed first principles calculations for on-surface and sub-surface oxygen on this surface. According to our calculations, the adsorption energies for all on-surface site oxygen atoms increase, whereas the energies of the sub-surface atoms decrease with the increasing oxygen coverage. At coverage 1 ML and higher on the reconstructed surface, structures including both on- and sub-surface atoms are energetically more favourable than structures consisting only of on-surface adsorbates. On the ideal (1 0 0) surface this change can be perceived at coverage 0.75 ML.     

Fuel cell and electrochemical studies of the ethanol electro-oxidation in alkaline media using PtAuIr/C as anodes

Ethanol electro-oxidation reaction was investigated considering conventional electrochemical experiments in alkaline media, direct ethanol fuel cell (DEFC), and in situ ATR-FTIR. The working electrode/anodes were composed of monometallic Pt/C, Au/C, Ir/C, and trimetallic PtAuIr/C nanoparticles with atomic Pt/Au/Ir ratios of 40:50:10, 50:40:10, 60:30:10, 70:20:10, and 80:10:10. X-ray diffraction (XRD) suggests PtAuIr/C alloy formation, and according to transmission electron micrographs, the mean particle sizes are from 4 to 6 nm for all catalyst compositions. PtAuIr/C 40:50:10 showed the highest catalytic activity for ethanol electro-oxidation in the electrochemical experiments; using this material, the peak current density from ethanol electro-oxidation on cyclic voltammetry experiment was 50 mA per g of Pt, 3.5 times higher than that observed with Pt/C. The fuel cell performance was superior using all PtAuIr/C compositions than using Pt/C. Au/C and Ir/C presented very poor catalytic activity toward ethanol electro-oxidation. The improved results obtained using PtAuIr/C might be related to the OH_ads species formed at low overpotential on Ir and to the decrease on adsorption energy of poisoning intermediates on Pt sites, promoted by Au.     

The structure of 55-atom Cu–Au bimetallic clusters: Monte Carlo study

We have investigated segregation phenomena in Cu–Au bimetallic clusters with decahedral structures at 100 K and 300 K, based on the second-moment approximation of the tight-binding (TB-SMA) potentials by using Monte Carlo method. The simulation results indicate that there are three regions (split, three-shell onion-like and core-shell region) at 100 K and two regions (split and core-shell) at 300 K with the structure of decahedral clusters, as the chemical potential difference Δμ changes. It is found that the structure of decahedral clusters undergoes a division into smaller clusters in the split region. In the core-shell structure, Au atoms are enriched in surface and Cu atoms occupy the core of the clusters because of the different surface energy of Cu and Au. The Au atoms are enriched in the surface shell, and the Cu atoms are in the middle shell, while a single Au atom is located in the center to form the three-shell onion-like structure. The structure and binding energy of smaller clusters after splitting are also discussed. The Au atoms generally lie on the surface of the smaller clusters after splitting.     

Morphological,Structural, and Compositional Evolution of Pt–Ni Octahedral Electrocatalysts with Pt‐Rich Edges and Ni‐Rich Core: Toward the Rational Design of Electrocatalysts for the Oxygen Reduction Reaction

The progress in colloidal synthesis of Pt–Ni octahedra has been instrumental in rising the oxygen reduction reaction catalytic activity high above the benchmark of Pt catalysts. This impressive catalytic performance is believed to result from the exposure of the most active catalytic sites after an activation process, chemical or electrochemical, which leads to a Pt surface enrichment. A foremost importance is to understand the structure and the elemental distribution of Pt–Ni octahedral, which leads to an optimal catalytic activity and stability. However, the factors governing the synthesis of the Pt–Ni octahedra are not well understood. In this study, unprecedented surface atomic segregation of Pt atoms in a Ni‐rich Pt–Ni octahedral nanoparticle structure is established by advanced electron microscopy. The Pt atoms are almost exclusively located on the edges of the Pt–Ni octahedra. This structure is formed in a pristine form, i.e., prior to any chemical or electrochemical etching. A new growth mechanism is revealed, which involves the transformation from an octahedron with a Pt‐rich core to a Ni‐rich octahedron with Pt‐rich edges. This observation may pave the way for a deeper understanding of this class of Pt–Ni octahedral nanoparticles as an electrocatalyst.